The present invention relates to a surface emitting semiconductor laser element, and, more particularly, relates to a surface emitting semiconductor laser capable of achieving fundamental transverse mode oscillation.
In a vertical cavity surface emitting semiconductor laser element (VCSEL; referred to simply as a surface emitting laser hereinafter), a resonation direction of light is perpendicular to a substrate surface, as the term indicates. The surface emitting laser has attracted a significant attention as a light source for communication, or a variety of devices in a sensor application.
The reason for the attention described above has been based on the following advantages. As opposed to a conventional edge-emitting laser, in the surface emitting laser, elements are easily arranged in a two-dimensional arrangement. It is not necessary to provide a cleavage for installing a mirror, so that a wafer-level test is possible. Further, an active layer has an extremely small volume. Accordingly, it is possible to oscillate the surface emitting laser at an extremely lower threshold, thereby reducing power consumption.
Especially, the surface emitting laser has an extremely small cavity length. Accordingly, it is possible to easily achieve fundamental mode oscillation in terms of a longitudinal mode of oscillation spectrum. On the other hand, the surface emitting laser does not have a control mechanism. Accordingly, the surface emitting laser tends to generate a plurality of higher order modes in terms of a transverse mode. When a laser beam oscillated with higher order transverse modes is used for optical transmission, a signal thereof is susceptible to degradation proportional to a transmission distance, especially during high-speed modulation. Thus, in the surface emitting laser, a variety of structures have been proposed as a measure for facilitating the fundamental transverse mode oscillation.
In order to obtain the fundamental transverse mode oscillation in a simple manner, an area of a luminescence region is reduced to an extent that only the fundamental mode can oscillate. For example, when the surface emitting laser has an oxidation constriction confinement structure including an AlAs layer selectively oxidized and an oscillation wavelength within an 850 nm band, a refractive index difference between a non-oxidized region and an oxidized region (Al2O3) in the AlAs layer becomes large. Accordingly, it is necessary to reduce an area of a luminescence region less than about 10 μm2 in order to achieve the fundamental transverse mode oscillation.
Generally, in the surface emitting laser having such an oxidization constriction confinement structure, a current constriction width, which controls an area of a luminescence region, is determined by an oxidized layer formed through selectively oxidizing a peripheral region of the AlAs layer. In order to form an aperture such that the area of the luminescence region becomes less than about 10 μm2 through forming the oxidized layer, it is necessary to precisely control the oxidization process, thereby lowering a product yield. In addition, when the area of the luminescence region is reduced, not only an output thereof is lowered but also an element resistance increases, thereby increasing a voltage applied to the surface emitting laser.
In the surface emitting laser, in order to increase the area of the luminescence region and achieve the fundamental transverse mode oscillation, a photonic crystal surface emitting laser has been proposed in a reference, for example, “IEEE Journal of Selected Topics in Quantum Electronics”, Vol. 9, No. 5, pp. 1439-1445, September/October 2003. FIG. 9 is a perspective view showing the conventional surface emitting laser disclosed in the reference, and FIG. 10 is a plan view thereof.
The conventional photonic crystal surface emitting laser 200 shown in FIG. 9 and FIG. 10 is designed to have an oscillation wavelength of 850 nm. In the photonic crystal surface emitting laser 200, a lower reflecting mirror structure 41 formed of 35 pairs of multilayer films in which p-type Al0.8Ga0.2As and p-type Al0.2Ga0.8As each of which has a thickness of λ/4n (where λ is an oscillation wavelength and n is a refractive index) are alternately laminated, is formed on, for example, a p-type GaAs substrate 40 using an MOCVD method. A lower cladding layer (not shown), a luminescence layer 42 constituted by 4 layers of GaAs/Al0.2Ga0.8As quantum well structure, and an upper cladding layer (not shown) are sequentially laminated on the lower reflecting mirror structure 41. Further, an upper reflecting mirror structure 43 formed of 25 pairs of alternately laminated n-type Al0.8Ga0.2As and n-type Al0.2Ga0.8As each of which has a thickness of λ/4n, is formed on the upper cladding layer. An n-type GaAs layer is formed on a surface of an uppermost layer of the upper reflecting mirror structure. In order to perform a selective oxidation process, a portion of the above structure up to at least a top surface of the lower reflecting mirror structure is removed by etching, and a mesa post 49, i.e., a layered structure with a column shape, is formed in a central portion.
Inside the mesa post 49 having the layered structure with the column shape, a two-dimensional hole arrangement is formed by electron beam lithography and dry-etching. The hole arrangement has a point defect region 32 with no hole formed therein in a central portion thereof. A period of an arrangement of circular holes 45 is, for example, 3 to 5 μm, and a depth of the circular holes 45 corresponds to that of 20 pairs of the upper reflecting mirror structure 43. An upper electrode 44 formed of, for example, AnGeNi/Au is formed on an outer periphery of the hole arrangement, and a lower electrode 47 formed of, for example, Ti/Pt/Au is formed on a backside surface of the substrate 40.
In the photonic crystal surface emitting laser mentioned above, a p-type AlAs layer is formed near an uppermost layer of the lower reflecting mirror structure 41 (i.e. near a luminescence layer). A mesa post outer periphery of the p-type AlAs layer is selectively oxidized to form an insulation region formed of Al2O3, whereby a current constriction structure 46 relative to the luminescence layer is formed.
The photonic crystal surface emitting laser 200 shown in FIG. 9 and FIG. 10 has a lamination structure in which the active layer (or the luminescence layer) 42 is disposed between the upper reflecting mirror structure 43 and the lower reflecting mirror structure 41 formed on the substrate 40. A reflectivity of the upper reflecting mirror structure 43 is lower than that of the lower reflecting mirror structure 41, so that a laser beam 48 is emitted through the upper reflecting mirror structure 43. A plurality of holes 45 arranged in the two-dimensional pattern is formed in the upper reflecting mirror structure 43, and in the central region of the two-dimensional pattern, a defect of hole, in which no hole is present, is provided. In the above-mentioned structure, a refractive index in the defect region where no hole is present is slightly greater than a refractive index in the peripheral region where the holes are arranged in the two-dimensional periodic pattern. With the slight difference between the refractive indexes, the region where the holes are arranged two-dimensionally acts as a clad while the central defect region where no hole is present acts as a core. Thus, the transverse mode control is performed based on a weak optical confinement, whereby an area of the luminescence layer in which only a fundamental transverse mode can oscillate is enlarged.
In the above-mentioned photonic crystal surface emitting laser, the laser oscillation occurs in the luminescence layer upon current injection. The oscillated laser beam is emitted through the upper reflecting mirror structure, having a lower reflectivity than the lower reflecting mirror structure. The laser oscillation occurs in a fundamental and single mode based on the transverse mode control mechanism defined by the slight difference between the refractive indexes of the central point defect region 32 and the peripheral two-dimensional hole arrangement region.
However, in the conventional photonic crystal surface emitting laser as shown in FIG. 9 and FIG. 10, which has the current constriction structure such as the oxidization constriction or the ion-implantation constriction, the two-dimensionally arranged holes are situated in the middle of a current injection path from the upper electrode 44 formed on the outer periphery of the hole arrangement region to the current injection region in the active layer. Accordingly, an electrical resistance of the element tends to increase greater than 100 ohms. Therefore, there are problems such that the light emission efficiency becomes small and a high voltage needs to apply. Furthermore, the manufacturing process of the oxidization constriction structure or ion-implantation constriction structure tends to be complicated, so that the adoption of an optimized structure of laser emission is restricted.